N. M. Konsevich, “Best Orthogonal Trigonometric Approximations of Classes of Functions of Many Variables $L^{ψ}_{β, p}$”, Ukr. Mat. Zhurn., vol. 53, no. 1, pp. 23–29, Jan. 2001, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4217